Optical communication systems offering very high data rates have been developed that take advantage of the wide bandwidth and low loss of optical fiber waveguides. For example, commercial communication systems have already reached per-channel data rates of up to 40 Gb/sec, while data rates of 160 Gb/sec and higher are routinely generated in research laboratories. Measurement and characterization of such high data rate signals continues to be challenging. Traditional optoelectronic measurement techniques in which a high-speed photodetector and a sample-and-hold circuit provide the required temporal gating function are generally inadequate at these ultra-high data rates. Nonlinear optical sampling based on mixing an optical data signal with a short sampling optical pulse in a nonlinear optical material can provide higher temporal resolutions due to the availability of short optical pulses. Unfortunately, nonlinear techniques exhibit limited sensitivity because of the low efficiency of the nonlinear optical mixing process. Thus, both conventional optoelectronic and nonlinear optical techniques are of limited value in optical applications that require signal monitoring and characterization at very high bit rates. For example, these conventional techniques are not well suited to optical component testing or adjustment.
A common diagnostic measurement for optical communication systems is the eye diagram. To form an eye diagram, temporally gated samples of a data stream are accumulated over a long time period. Temporal gating is based on a series of sampling strobe pulses, and is implemented at a bandwidth comparable to or greater than the bandwidth associated with the data rate of the data signal being measured. The sampling frequency (the rate at which samples are acquired) can be several orders of magnitude lower than the data rate. In an eye diagram, signal samples are accumulated and displayed based on their relative positions within a periodic bit slot. The resulting eye diagram provides information regarding noise and distortions present in the data signal. Signal averaging of the signal samples is not generally useful because the signal samples within any selected periodic bit slot include samples of both “1” and “0” bits as well as transitions between “1” and “0.” In optoelectronic-based eye diagram measurements, the temporal gating window is limited by available electrical bandwidths. In addition, electronic sampling oscilloscopes offer limited sensitivities to weak electrical signals produced by many optical signals. Thus, optoelectronic based eye diagram measurements exhibit inadequate temporal resolution and inadequate sensitivity.
Nonlinear, all-optical measurement techniques can take of advantage of ultrafast optical sources such as mode-locked lasers and fiber-grating compressors. These sources can provide very short optical pulses (less than a ps) at relatively low repetition rates (a few MHz). In nonlinear optical sampling, these short optical pulses are used as sampling (gating) pulses, and an optical data signal can be characterized based on a nonlinear interaction of a sampling pulse with the optical data signal in a nonlinear optical material. For example, an interaction of the sampling pulse and the data signal can produce an optical measurement signal that is proportional to a product of the peak powers of the data and sampling pulses. Because nonlinear optical processes are generally inefficient, high peak powers of the sampling and/or data pulses are generally needed. Thus, these nonlinear optical techniques are associated with higher costs and greater complexity in the sampling source or in the signal under test because additional optical amplification is often needed. Even at high peak powers, these techniques have efficiencies that are undesirably low.
In view of these and other limitations, improved methods and apparatus for optical signal characterization are needed.